1. Field of the Invention
The present invention relates to a hydraulic circuit for hydraulic construction machines such as a hydraulic excavator, and more particularly to a hydraulic circuit for construction machines for driving an actuator for a working machine or an actuator for a working element by joining hydraulic fluid flows of a plurality of hydraulic pumps.
2. Description of the Prior Art
A hydraulic construction machine and a hydraulic circuit used therein according to the prior art will now be described with reference to FIGS. 1 and 2.
FIG. 1 is a side elevational view showing an example of an excavator in which a nibbler is used as a working element instead of a bucket.
The hydraulic excavator shown in FIG. 1 includes a track 102 travelled by a hydraulic motor 101, a swing 104 mounted on the track 102 and rotated by a swing motor 103, a boom 106 rotatably mounted on the swing 104 and driven by a hydraulic cylinder for boom 105, an arm 108 rotatably mounted on the boom 106 and driven by a hydraulic cylinder for arm 107, and a nibbler 301 rotatably mounted on the arm 108 and rotated by a hydraulic cylinder for bucket 109. Reference numeral 111 denotes an operation lever provided within a cab and for driving the hydraulic cylinder for arm 107, for example. The bucket, the nibbler and the like, and the arm and the boom will hereinafter be respectively referred to as a working element and a working machine.
Generally, the hydraulic circuit shown in FIG. 2 is used in the hydraulic excavator that uses the nibbler 301 as the working element. A pilot operating valve 201 is connected to the operation lever 111. The pilot operating valve 201 is adapted to adjust a pressure of the hydraulic fluid from a pilot pump 202 in accordance with an operating amount of the operation lever 111, thus changing a first directional control valve 1. Reference numeral 50 denotes a first hydraulic pump that is adapted to supply the hydraulic fluid to a first directional control valve group 51 including a directional control valve 54 for the swing motor 103 and the first directional control valve 1 for the hydraulic cylinder for arm 107 disposed downstream of the directional control valve 1. The directional control valve 1 is provided with a first output port 3 connected through a passageway 31 to a bottom side chamber 30 of the hydraulic cylinder 107, a first input port 8 to which the hydraulic fluid is supplied through a check valve 6 from a center-bypass passageway, a second output port 4 connected through a passageway 33 to a rod side chamber 32 of the hydraulic cylinder 107, and a second input port 9 to which the hydraulic fluid is supplied through a check valve 7 from a parallel-passageway 55. A restrictor 10 is interposed in a passageway connecting the first and second input ports 8 and 9 together. The restrictor 10 is for operating the swing motor 103 and the hydraulic cylinder for arm 107 at the same time in favorable manner. When an arm crowding operation (arm lowering operation) of which hydraulic pressure is low and a swing operation of the swing motor 103 of which hydraulic pressure is high are performed at the same time, the restrictor 10 prevents a lot of the hydraulic fluid from flowing into the bottom side chamber 30 of the hydraulic cylinder for arm 107 of which hydraulic pressure is low, so that the hydraulic fluid is supplied to the swing motor 103 of which hydraulic pressure is high. Thus, it is prevented that only the arm crowding operation is performed and the swing operation is not performed.
When an arm damping operation (arm raising operation) of which hydraulic pressure is high and the swing operation are performed at the same time, the hydraulic fluid is supplied to the rod side chamber 32 of the hydraulic cylinder for arm 107 from the parallel-passageway without passing through the restrictor 10 and further when only the arm crowding operation is performed, the hydraulic fluid is supplied to the bottom side chamber 30 from the center-bypass passageway without passing through the restrictor 10. Accordingly, the operation of the hydraulic cylinder for arm 107 is satisfactory.
Reference numeral 52 denotes a second hydraulic pump for supplying hydraulic fluid to a second directional control valve group 53 including a second directional control valve 2 for joining the hydraulic fluid. The directional control valve 2 is provided with an input port 11, a first output port 22 connected to a rod side chamber 81 of the hydraulic cylinder for nibbler 302, and a second output port 5 connected to a bottom side chamber 82 thereof. The directional control valve 2 is changed over simultaneously with the changing over of the directional control valve 306 by means of a pilot valve 303 driven by an operation lever 304, thereby joining the hydraulic fluid of the second hydraulic pump 52 with the hydraulic fluid of the first hydraulic pump 50 and thereby providing the joined hydraulic fluid to the hydraulic cylinder 302. In such a hydraulic circuit, the joined hydraulic fluid of the first and second hydraulic pumps 50 and 52 is supplied to the hydraulic cylinder for nibbler 302 but only the hydraulic fluid derived from the first hydraulic pump 50 is supplied to the working machine actuator (the hydraulic cylinder for arm 107 is shown by way of example in FIG. 2). This would deteriorate the workability of the arm 108. This is because in the ordinary hydraulic excavator that uses a bucket as a working element, the hydraulic cylinder for arm receives the hydraulic fluid from the first and second hydraulic pumps 50 and 52.
Also, when it is desired to carry out simultaneously the arm damp operation (i.e., raising operation) and the operation of the nibbler 301, in the case where there is no object to be gripped by the nibbler 301 or it is then necessary to release the gripping action of the nibbler 301, the hydraulic pressure for the nibbler 301 is low relative to that of the arm 108. As a result, the hydraulic fluid derived from the hydraulic pump 50 would flow to the hydraulic cylinder for nibbler 302 but would not be supplied to the hydraulic cylinder for arm 107. For this reason, there is a disadvantage in that it is impossible to carry out the damp operation of the arm 108 simultaneously with the operation of the nibbler 301.